YA, as you probably know, stands for young adult. The general definition for YA is teenage main characters with coming of age themes. I would say that Fools Gold meets that.
NA, a newer category you may not know, is new adult. A common definition of that is main characters slightly older than YA who are on their own for the first time and exploring situations such as college, new jobs, and serious relationships. I would say Fools Gold meets that too. Of course in 1898, Ellie Webster had to grow up more quickly and become an adult at an earlier age than some young people today. There was no high school, and college was not really a possibility for her. So her reality was different than many typical NA main characters. Most NA novels are contemporary and sexy, so my book doesn’t fit in the usual perception. But it deals with many of the same things, in a lighter way. Ellie would definitely tell you she’s not a teenager, or even remotely, that she’s an adult. And if she were here today she’d probably prefer the NA category.
I usually tell people I write YA historical romance. That’s enough adjectives for a mouthful. But I would say that I write YA/NA when I have the room to write the whole thing. Whatever label you give it doesn’t really matter. The big thing is the story.